Many types of pneumatic medical devices have an inflatable part attached externally to a patient and an instrument part connected by tubing to the inflatable part for controlling the pressure in the inflatable part in order to continuously or intermittently apply a constant pressure or a desired pressure waveform to the portion of the body beneath the inflatable part. Two common type of devices are pneumatic tourniquet systems for surgery and limb compression systems for the prevention of deep vein thrombosis and treatment of lymphedema.
In a typical pneumatic tourniquet system, an inflatable cuff encircles a limb at a desired location and is connected pneumatically by flexible pneumatic tubing to a pressure controller which maintains the pressure in the cuff above a minimum pressure required to stop arterial blood flow in the limb distal to the cuff over a time period suitably long for the performance of a surgical procedure distal to the cuff location. Many types of such pneumatic tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. Nos. 4,469,099, No. 4,479,494 and No. 5,439,477, and by McEwen and Jameson in U.S. Pat. App. 08/297,256 filed on Aug. 26, 1994.
In a typical limb compression system for the prevention of deep vein thrombosis and treatment of lymphedema, an inflatable appliance is attached to a limb and is connected by flexible pneumatic tubing to a pressure controller which controls the pneumatic pressure in the appliance to periodically inflate the appliance and thus periodically apply pressure to the underlying limb and augment the flow of venous blood proximally in the limb. Examples of such limb compression systems are given in U.S. Pat. No. 3,892,229 of Taylor et al. and in U.S. Pat. No. 4,013,069 of Hasty. Another example is given in pending U.S. Pat. App. 08/639,782 of McEwen and Jameson, filed on Apr. 29, 1996.
A common problem associated with the use of pneumatic tourniquet systems and limb compression systems relates to the flexible pneumatic tubing which establishes a pneumatic pathway between the pressure controller and the pneumatic cuff or appliance. The pressure controller is often located remotely from the patient, necessitating the use of long and very flexible tubing extending from the controller and around and between staff and other equipment to the patient. Pneumatic connectors are typically used to connect and disconnect the flexible tubing from the pressure controller, and from the cuff or appliance. During usage of tourniquet systems and limb compression systems, the patency of the tubing including the associated connectors, or the degree of pneumatic obstruction produced by the tubing, can change and seriously impair the function of such systems. For example, before or during use the tubing can become kinked and partially or completely obstructed, thus restricting pneumatic flow or completely isolating the pressure controller from the cuff or appliance, and thus preventing a desired pressure from being produced in the cuff or appliance. Also, through malfunction or operator error, the tubing can become disconnected from the pressure controller or from the cuff or appliance, again preventing a desired pressure from being produced in the cuff or appliance.
Birmingham et al. describe in U.S. Pat. No. 4,520,819 a tourniquet system with differential pressure occlusion detector for detecting certain types of tubing obstructions, but this invention is restricted to tourniquet systems which have two pneumatic tubes between the pressure controller and the pneumatic cuff, and the invention has other limitations.
The present invention incorporates an adaptation of the principles of acoustic reflectometry: by introducing a pneumatic pressure pulse of short duration into a pneumatic conduit and then analyzing reflections arising from the pressure pulse which occur when the pulse encounters a change in the cross sectional area of the conduit as it propagates throughout the conduit, by comparing the amplitude of the introduced pulse to the amplitude of the reflected pulse the pneumatic conduit may be characterized in terms of cross-sectional area and length. Such pneumatic conduits may be characterized in terms of cross-sectional area and length because any difference in the cross-sectional area at a particular location along the length of the conduit is proportional to the amplitude of the acoustic pulse reflected from the location, and the time delay until the reflected pulse is detected can be analyzed to determine the particular location along the length of the conduit. A more detailed explanation of the principle of acoustic reflectometry is included in U.S. Pat. No. 4,326,416 of Fredberg.
Techniques have been described in the prior art employing acoustic reflectometry to determine physical characteristics of passageways in living subjects, such as the airway, from measurements made at the mouth. See, for example, U.S. Pat. No. 4,326,416 and re-examination certificate B1 4,326,416 to Fredberg. Other techniques have been described in the prior art employing acoustic reflectometry and a wave tube in the nasal cavity of a subject to determine the shape of the nasopharyngeal cavity. See, for example, U.S. Pat. No. 5,316,002 to Jackson et al. In U.S. Pat. No. 5,445,144, Wodicka et al. described a technique employing acoustic reflectometry to guide the placement, determine position, and insure patency of a moveable tube or catheter within the body. The prior art does not describe techniques or apparatus for detecting or locating a partial obstruction, a complete obstruction, or a disconnection of tubing and associated connectors communicating pneumatically between a pressure controller and an inflatable cuff or sleeve of a pneumatic medical device attached externally to the body of a living subject at a fixed location and pressurized periodically or for an extended time period to achieve a therapeutic purpose.